In the manufacture of semiconductor devices, typically thousands of individual transistor devices are formed upon a single silicon substrate. These devices are interconnected to form complex circuits, also known as integrated circuits, as required for a particular circuit design. Because the transistors are formed within the same substrate, the transistors must be electrically isolated except as interconnected according to the circuit design; otherwise, undesired electrical connections between the transistors would cause circuit shorts. Several methods exist for device isolation and vary with the type of device being manufactured. One device isolation method widely used in the manufacture of insulated gate field effect transistors (IGFET) is the well known localized oxidation of silicon, or LOCOS, process.
In a typical LOCOS process, a thin silicon oxide layer, or pad oxide, is grown over a silicon substrate, and then a silicon nitride layer is deposited over the silicon oxide layer. Next, the pad oxide and nitride layer are patterned by known photolithographic techniques to partially expose the substrate. The exposed regions of the substrate are known as the field regions. Regions of the substrate still covered with the pad oxide and nitride are known as the active regions and will eventually contain the transistors for the integrated circuit. A thick silicon oxide insulator, or field oxide, is grown in the field regions of the silicon substrate by placing the substrate in a steam ambient, typically at a temperature in the range of 900.degree. to 1100.degree. C., for an extended time. The steam reacts with the exposed silicon to form silicon oxide. The thick field oxide provides electrical isolation by increasing the threshold voltage in the field region, thereby preventing the formation of a conductive path in the surface of the underlying silicon substrate. The active regions remain unoxidized and covered by nitride during the field oxide growth. Finally, the nitride and pad oxide are removed. Transistors are then formed by additional processing in the active region.
The continuing trend in integrated circuit design is to further increase the packing density of active devices on the silicon substrate. The density of active devices can be increased by shrinking some or all of the dimensions of the devices. One approach for increasing the packing density is the reduction of lateral separation distance between the active regions. However, as the lateral separation distance is reduced, the electrical isolation of adjacent active regions becomes more difficult. While the conventional field oxide structure is adequate to provide electrical insulation between adjacent active regions when the device separation distances are quite large, additional measures must be taken to ensure electrical isolation as the separation distances are reduced.
A well known technique for increasing the electrical isolation capability of a field oxide region is to place a doped region in the substrate below the field oxide region. This doped region is known as a channel-stop. The channel-stop functions to prevent electrical current from traversing between adjacent active regions below the field oxide. Typically, the channel-stop region is formed by implanting dopant atoms into the substrate region after patterning the nitride and pad oxide layers, and prior to growing the field oxide. In this way, the dopant is placed in the same locations of the substrate in which the field oxide regions will be formed. Although the dopant atoms are placed into the substrate using the patterned nitride and oxide layers as a doping mask, the dopant atoms can laterally diffuse through the substrate during the oxidation process used to form the field oxide regions.
The lateral diffusion process has the effect of dispersing the dopants in the substrate. At the elevated temperatures necessary to grow the field oxide regions, considerable lateral dopant diffusion can occur under the patterned nitride and pad oxide. Once the field oxidation process is complete and the nitride layer and pad oxide layer are removed, the active region of the substrate now contains a considerable dopant concentration from the lateral diffusion of implanted dopants. Additionally, the initial implanted concentration of dopants in the substrate region beneath the field oxide layer is reduced by the segregation of boron into the growing oxide layer. If the dopant concentration in the channel-stop region becomes too low, the channel-stop will not be able to perform its electrical current blocking function. This can lead to device failure from electrical coupling between adjacent active regions, known as punch-through. Although the channel-stop and field oxide structures remain viable isolation techniques, improvement in the isolation process is necessary to ensure adequate electrical isolation as the separation distances between adjacent active regions are reduced.